Salut !
I do not think there is one available for the default kobo device (i may be wrong).
If this is true, you still have an option to use a japanese dictionary on top of koreader.
I always use koreader so I did not even try to find a dictionary with the "default" kobo mode.
Anyways:
- koreader:
https://github.com/koreader/koreader...n-Kobo-devices
- japanese dictionary:
https://github.com/koreader/koreader...panese-Support
It will give you an experience similar to what you may experience with the chrome/firefox yomichan plugin.